The concept of a tension actuator which contracts along its longitudinal axis when inflated is known. Such an actuator, which responds at relatively low fluid pressure, is disclosed in U.S. Pat. No. 3,645,173--Yarlott. The disclosure of Yarlott specifies a number of parameters which are markedly different from or contrary to the present invention as will be pointed out in or will become understood from the specification considered in conjunction with the accompanying drawings. In Yarlott's tension actuator:
(A) The surface area of the shell remains substantially constant in all of the various positions of the actuator. A two-way network of relatively inextensible strands,--(i) extending axially, and (ii) helically wound causes the reinforced shell to "resist elastic expansion". In other words, this reinforcing network in Yarlott's actuator is attempting to maintain substantially constant surface area in all deformed positions. However, the elastomeric shell wall must necessarily undergo a shearing deformation as the actuator is inflated for causing it to contract. This shearing of the elastomeric shell wall causes a basic incompatibility at the junction where the shell wall is attached to the rigid cylindrical coupling members at each end. Because of this shearing of the shell wall, the cylindrical end members must be of small diameter in the Yarlott actuator in order to minimize the basic incompatibility, which restricts the fluid flow through them and thus inherently slows the cycle time, i.e. causes a slow response to changes in pressure. If an attempt is made to enlarge the diameter of these cylindrical end members, in order to speed up the response time, then the basic shear versus non-shear incompatibility at the shell-to-end-member junction is accentuated leading to large localized stresses and early failure of the shell wall at this junction.
(B) The Yarlott tension actuator is particularly adapted for low pressure applications, for example, pressures in the nature of 0.25 p.s.i. gauge up to a practical limit of about 15 p.s.i. gauge; that is, up to a limit of about one atmosphere of pressure difference between internal fluid pressure and ambient pressure.
(C) The Yarlott tension actuator has extreme sensitivity to internal fluid pressure exceeding 15 p.s.i. gauge, because above that limit the elastic shell begins to expand unduly by locally bulging between the axial and helical strands, but no further axial contraction actually occurs, leading to rapid fatigue failure and likelihood of bursting when cyclically operated for more than a relatively few cycles with repeated internal pressure excursions much above 15 p.s.i. gauge. In summary, the kind of tension actuator as invented by Yarlott within its normal limited low pressure range produces a minimum of stretch of its elastic shell with a maximum of bending and flexing of the shell and considerable shear deformation of the shell near its end member connections. On the other hand, single-crossing hyperboloidal tension actuators embodying the present invention are the opposite. They do intentionally involve considerable shell stretch, and they are able to operate for hundreds of thousands of cycles with each cycle involving a pressure excursion from about 0 p.s.i. gauge up to about 30 p.s.i. gauge and back to about 0 p.s.i. gauge without any apparent significant fatigue effects.
Another device which axially contracts upon inflation is disclosed in U.S. Pat. No. 2,642,091--Morin. However, the Morin diaphragm suffers from the problem that in its neutral (deflated) state it has the geometrical configuration of a right circular cylinder, more commonly called a cylindrical surface of revolution, with inextensible threads each placed along a generating line (axially extending straight line) or each along a helix with constant pitch. Consequently, a very large increase in internal fluid pressure is needed to be applied within the Morin actuator before its reinforced hose-like wall begins to bulge for causing axial contraction.
Furthermore, if the helical threads have a pitch of 52.degree., and if the Morin actuator is sufficiently long that these threads make at least one complete turn (at least one complete convolution) from end to end of this cylinder of revolution, then mathematical analysis shows that no effective axial contraction will take place regardless of how high is raised the pressure of the internal fluid. In other words, even if the internal pressure in such a hose is raised to the bursting point, no significant axial contraction will occur. In summary, the Morin structure makes inefficient use of materials and causes relatively large internal stresses and strains without producing a proportional contraction in its axial length. In contrast, a tension actuator constructed in accordance with the present invention produces a much longer and more forceful contraction (longer and more forceful stroke) with the same materials and the same changes in internal fluid pressure.
U.S. Pat. No. 3,638,536--Kleinwachter et al discloses diaphragm devices for transforming a fluid pressure into torsional movement or into axial movement upon inflation. The diaphragm is elastically stretchable in preferably only one direction.
U.S. Pat. No. 2,789,580--Woods discloses a two-component mechanical transducer with an expansible cavity formed by a flexible seal having a cylindrical braided or woven metal sheath encompassing it. There is the undesirable complexity of an outer cylindrical braided sheath and a separate internal pressurizing means. An actuator embodying the present invention is a substantial simplification over the Woods' device, by virtue of being a one-component structure as distinguished from Woods' two-component structure.
U.S. Pat. No. 2,865,419--Cunningham has been reviewed by the present inventor and is considered even more remote from the present invention than the above-listed disclosures. The Cunningham structure exploits the neutral helical braid pitch of approximately 52.degree. (as discussed above in connection with Morin's disclosure) in order to yield a dimensionally stable structure, i.e. a structure which will neither expand nor contract nor change radius upon changes of pressure in the internal fluid. This Cunningham reference is set forth as being known to the inventor in order for this discussion of known disclosures to be complete and in the event the reader might consider it to be of interest. This Cunningham patent does support the earlier explanation that a hose-like structure reinforced with two-way helical strands at a pitch of 52.degree. and each extending for at least one full convolution is dimensionally stable; therefore, such structure has exactly the opposite characteristics from the desired long and strong stroke, fast-response axial contraction characteristics needed in high efficiency tension actuators as provided by the present invention.